BEARING BALLS FOR OFFSET CENTERS

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BEARING_BALLS_FOR_OFFSET_CENTERS.jpg

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This is an idea that sprung to my mind a while ago, although I'm 
sure I'm not the first one to think of it.

When offsetting the tailstock for taper turning, or using a special 
tailstock fixture for the same purpose, the 60 degree center points don't 
fit well in the centerholes of the work being taper turned.

This method needs custom-made lathe centers for both headstock and tailstock.  
The sharp point is turned off for a short distance, and centerdrilled just 
as is done for the work being turned.

Hardened steel balls are captured in the centerholes between the lathe 
centers and the work, at each end.

The correct centerhole size is important in relation to the bearing ball diameter.

For a standard 60 degree centerdrill, the opening of the hole at the ends 
should ideally be between 88% and 90% of the diameter of the ball.

If larger, there may not be enough clearance between the lathe center 
and work to allow any offset.
If the hole's opening is smaller than 87% of the ball's diameter, only 
the corner of the hole's opening will contact the ball and the whole 
thing may come loose under heavy cutting pressure.

In practical experience, I've had very good results with this technique 
while turning morse taper shanks.

For the purpose of accurately setting the tailstock setover, the effective 
length of the workpiece is measured between the centers of each ball. 

Just mike the workpiece with the balls in place, and subtract the total of 
one half the diameter of each ball.

Be sure to use your favorite tailstock center lube on that end 
(I use white lithium grease).

Hope this is useful,

Ken Grunke
West Lima, WI
Jan. 09, 2005